1. Field of the Invention
The present invention relates to a semiconductor ceramic element and a method for producing the semiconductor ceramic element, and more particularly, to a semiconductor ceramic element which has combined electrical characteristics including PTC characteristics and NTC characteristics, and a method for producing the semiconductor ceramic element.
2. Description of the Related Art
Typical applications of thermistors include temperature sensors. Temperature sensors using thermistors have different resistance-temperature characteristics required depending on the applications, and thus, there has been a need for the temperature sensors to have an ability to provide a variety of resistance-temperature characteristics. For example, Japanese Utility Model Application Laid-Open No. 63-75001 or Japanese Patent Application Laid-Open No. 7-167715 proposes a thermistor temperature sensor which has a PTC element and an NTC element combined in order to develop required resistance-temperature characteristics.
While there is a possibility that various resistance-temperature characteristics will be achieved when the PTC element and the NTC element are combined as described above, the PTC element and the NTC element are not able to simply have a structure with the elements bonded to each other with a common electrode interposed therebetween when attention is drawn to the bonded section between the PTC element and the NTC element. This is because the reliability of the junction may be decreased in some cases, or the electrode material which can make ohmic contact with the PTC element and the electrode material which can make ohmic contact with the NTC element may be different from each other in some cases.
Therefore, in Japanese Utility Model Application Laid-Open No. 63-75001, a structure which has electrodes respectively provided on a PTC element and an NTC element and bonded to each other is used. However, the use of this structure increases the number of sections for bonding different elements, such as the bonded section between one electrode and the other electrode, the bonded section between the PTC element and the electrode, and the bonded section between the NTC element and the electrode, which increases the probability of peeling at the bonded sections.
On the other hand, in Japanese Patent Application Laid-Open No. 7-167715, a structure which has a PTC element and an NTC element bonded to each other simply with one common electrode is used. However, this common electrode has to be made of a special material such as a material which guarantees the contact of temperature-dependant resistance by barrier layer destruction, and forms a diffusion barrier against interdiffusion of each of the sections constituting the thus contacted temperature-dependant resistance into the other temperature-dependant resistance. In addition, Japanese Patent Application Laid-Open No. 7-167715 also fails to prevent the problem of peeling as in the case of Japanese Utility Model Application Laid-Open No. 63-75001, as long as there is any electrode at the bonded section between the PTC element and the NTC element.